Brakes of this type find application, for example, in brake-by-wire brake systems. With such brake systems, direct or immediate access to the brakes by the driver is not possible and is also not desirable. Whereas with usual hydraulic brake systems the driver displaces hydraulic brake fluid into the brakes by actuating the brake pedal, whereby the service brakes are actuated and the vehicle is braked, with brake-by-wire brake systems actuation of the brake pedal, and actuation or active activation of the brakes, are decoupled from one another. With such brake systems, the driver actuates an actuation unit in which the pedal travel is measured by a sensor, for example a pedal travel sensor. This information is then utilized in an electronic control unit to send an application command to the actively activatable brakes. With electromechanically actuatable brakes or electromechanical brakes (EMB), an electric motor is activated for this purpose, its rotary motion being converted by an actuator via a transmission into a translational motion by which, in the case of disk brakes, for example, the brake pads are pressed against the brake disk.
In brake-by-wire systems, comfort functions such as a parking brake functionality which holds the vehicle when parked on an incline, and safety functions such as ABS, can be implemented in a convenient manner. In an ABS control process, for example, the direct decoupling of brake pedal from actuation of the service brakes eliminates the “pumping” of the brake pedal known with hydraulic brake systems.
An important safety criterion or safety-relevant design criterion for brake systems and their associated brakes is the implementation of a fall-back level in the event that important components fail. For example, in hydraulic systems with a brake servo it must be ensured that, in the event of failure of the brake servo, the driver can nevertheless decelerate the vehicle to a sufficient degree by stronger actuation of the brake pedal.
In addition, it must be ensured for safety reasons that the vehicle remains maneuverable in the event of failure of the on-board network. A dynamic situation which must be avoided in all cases is one in which the brakes remain applied in the event of such a system failure. This is especially relevant with brake-by-wire systems. If, for example, the driver has actuated the actuation unit, whereby the electromechanically actuatable brakes have been applied, and if the on-board network now fails, it must be ensured that the brake pads are released again from the corresponding brake disks. If this does not happen the vehicle becomes difficult or even impossible to control and can go into a skid, especially when the road surface has a low coefficient of adhesion.
In order that the brake pads are released from the brake disks in such a case of on-board network failure, a corresponding mechanism which performs this function should be present in an EMB. Such a mechanism must not be dependent on a voltage supply from the on-board network since this is just what is not available in such a situation. The mechanism should serve to release the brake pads precisely in the currentless state of the brake by means of a separate current supply or by mechanical means.
Consequently, electric brakes must switch in the event of failure to a safe state free of braking torque. This requirement must be fulfilled in a very short time (50 ms for front wheel brakes, approximately 500 ms for rear wheel brakes) in order that the lateral grip of the wheel concerned, and therefore the steerability and stability of the whole vehicle, are not dangerously impaired. As long as the drivetrain of the electric wheel brakes continues to be activatable in a controlled manner, freedom from braking torque can be obtained by active retraction of the brake pad in the event of a critical fault, for example failure of the force sensor. If, however, the energy path to the wheel brake actuator is interrupted by a fault in the on-board network, in the electronic control system or in the electric motor itself, or if failure of the drivetrain is present for another reason, the brake must be configured to be self-opening without an external energy supply, that is, in a currentless state, and must react according. The “currentlessly self-opening” property makes the brake intrinsically safe and, in the event of software faults, also makes possible advantageous external switching-off by an external control device which causes interruption of the current supply to the wheel brake.
If no electric energy supply is available, the safe, braking-torque free state must be attained using the mechanical energy (expansion of the brake caliper) introduced into the system during the braking process. Since low mechanical efficiency may arise in the back-rotating electromechanical system under unfavorable conditions (for example, low ambient temperatures or increased friction through wear), in the event of failure either an excessive residual braking moment is therefore applied to the wheel, or the time needed to fall back below a given limit value is too great.
It is therefore necessary to implement an independent energy store which makes available a certain minimum quantity of energy in the event of failure in order to make possible reliable opening of the brake within a prescribed time.
With known electromechanical brakes, the energy stored in an energy store which is independent in this way cannot automatically or autonomously adjust itself to the wear situation of the linings or brake pads, or the adjustment mechanism is configured in such a way that the spindle must be rotated through a given angle before automatic opening of the brake in the event of failure can be reliably implemented. An electromechanically actuatable disk brake in which releasing of the brake pad from the brake disk is effected by a spring energy store in the currentless state of the electric motor is known, for example, from DE 198 07 328 C2.